1. Field of the Invention
This invention relates generally to electroacoustic resonating apparatus and more specifically to apparatus for generating resonances of tunable frequencies.
2. Description of the Prior Art
Stable, low noise oscillators are a vital element in radar systems for their ability to provide coherent synchronization between the radar's transmitted signals and received signals. To provide this function, the oscillator frequency must be highly stable, with low noise spectral power density.
In a typical system, the output of a low noise quartz oscillator at 80 MHz is multiplied in frequency to the desired radar carrier frequency. The phase noise at greater than 10 KHz from the carrier frequency can be further reduced by use of a very high Q resonator, such as an HBAR (high overtone bulk acoustic resonator) which have shown Q's of greater than 50,000 at 1.5 G.Hz.
Typically, the low frequency (80 MHz) quartz oscillator is designed for lowest phase noise and not necessarily low frequency drift. Thus, the frequency drift of the low frequency oscillator output may exceed the bandwidth of the HBAR filter and the noise filtering will not be effective. It is desirable that the frequency of the HBAR filter can "track" the frequency drift of the low frequency oscillator so that the radar carrier frequency is always in the center of the HBAR passband.
This tracking is achieved by use of a tunable HBAR, connected into an automatic frequency control loop. Use of a tracking filter also allows any manufacturing tolerances to be automatically tuned out.
In the prior art, the sole means of controlling the resonance frequencies of the high overtone bulk acoustic resonator is to vary the temperature of the resonator environment. A correlation between temperature and the resonance frequencies of an acoustic resonator was described in an article entitled "Temperature Compensated Bulk Sheer Microwave Resonators in LiTaO.sub.3 " in Applied Physics Letters Aug. 15, 1984 by B. R. McAvoy, co-inventor of the present application and S. V. Krishnaswamy. The teachings of this article are hereby incorporated by reference into the present application. This correlation allows tuning of the resonator by means of increasing or decreasing the temperature of the resonator environment. However, tuning by this means typically requires several seconds, a time period too long for use of temperature controlled high overtone bulk acoustic resonators in radar system applications.
U.S. Pat. No. 4,573,027 dated Feb. 25, 1986, entitled "Bulk Acoustic Resonator Tracking Filter," inventors Michael S. Buchalter, Francis W. Hopwood and James T. Haynes teaches an acoustic resonator not operable to be turned magnetically.
For the high overtone bulk acoustic resonator described hereinabove, a typical frequency response is shown in FIG. 1. The resonance frequencies, f.sub.n, are defined by the equation ##EQU1## where "c" is the velocity of the wave through the substrate, "a" is the length of the substrate and "n" is an integer typically of the order of 10.sup.2. For each value n there will be a resonance associated therewith. The resonances repeat periodically with a period of "c/2a".
Changes in temperature of the high overtone bulk acoustic resonator cause the resonance frequencies of the acoustic resonator to drift to higher or lower frequencies. Experimental results indicate a parabolic dependence between the temperature and the resonance frequencies.
Shown in FIG. 2 is the relationship between the frequency of an acoustic wave and the wave number "k" defined by EQU k=(2.pi./.lambda.)
where ".lambda." is the wave length of the acoustic wave. The relation between "k" and "f" is linear and may be described by the expression f=ck; where "f" is the frequency of the acoustic wave, "k" is the wave number, and "c" is the velocity of the wave through the medium.
When acoustic waves are caused to propagate through a ferrimagnetic material, the relationship between the frequency f, and the wave number "k" is no longer purely linear due to a magneto-elastic interaction with spin waves. The spin waves are caused by exchange coupling between successive ferrimagnetic atoms, and are defined by the equation: EQU (f/.gamma.)=H+Dk.sup.2
where "f" is the frequency of the spin wave, ".gamma." is a gyromagnetic ratio (2.8 MHz/Oe), "H" is the magnitude of the magnetic field intensity in the ferrimagnetic material (Oe), "D" is an exchange constant and k is the wave number (cm.sup.-1). Such a relationship is shown in FIG. 3. By altering the magnitude of the magnetic field intensity "H", the spin wave may be translated up or down in frequency.
In FIG. 4 the curves representing the acoustic wave and the spin wave are plotted simultaneously. The two curves cross over, the particular frequency of which is determined by the magnitude of the magnetic field intensity "H". At this point where the wave lengths and frequencies of the two waves are equal, coupling between the waves occurs. The resultant relation called the dispersion relation, is expressed graphically in FIG. 5.
At lower wave numbers, k, the frequency of the wave is in the acoustic region 10 of the curve wherein the relationship between f and H is linear. At high wave numbers, k, the frequency of the wave is in the magnetic region 12 wherein the relationship between f and k is quadratic. Most importantly, for certain values of wave numbers k, the frequency of the wave is in a third region which is neither purely acoustic nor purely magnetic. This third region, the magneto-elastic region 14, exhibits characteristics of both the acoustic region 10 and the magnetic region 12. For a given wave number in the magneto-acoustic region 14, then, the associated resonance frequency is different than for the pure acoustic wave of FIG. 2 or for the pure magnetic spin wave of FIG. 3. By altering the magnitude of the magnetic field intensity, the location of this magneto-elastic region 14 may be altered. This effectively allows fine tuning of the frequency location of a resonance for a given wave number k.